Electron gun



April 5, 1960 N. c. CHRISTOFILOS ET AL 2,931,939

ELECTRON GUN 2 Sheets-Sheet 1 Filed June 24, 1958 0 TRIGGER ENERATOR HALF WAVE FILAMENT POWER SUPPLY POWER SUPPLY PULSED f POWER 72 SUPPLY INVENTORS. NICHOLAS C. CHRISTOF/LOS KENNETH W. EHLERS A T TORNE Y April 5, 1960 c c s o os ETAL 2,931,939

ELECTRON GUN 2 Sheets-Sheet 2 Filed June 24, 1958 INVENTORS. NICHOLAS C. CHRISTOF/LOS KENNETH W. EHLERS ATTORNEY ELECTRON GUN Nicholas C. Christofilos, Berkeley, and Kenneth W. Ehlers, Lafayette, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Application June 24, 1958, Serial No. 744,297

g 5 Claims. (Cl. 315-) The present invention relates generally to the production of energetic electrons, and more particularly to a high energy, high current pulsed electron gun.

Electron guns and other sources of electrons are well known in the fields of electronics and nuclear physics for producing a pulsed beam of energetic electrons. Such sources generally include an electron emissive cathode and suitable accelerating electrode structure disposed in a low pressure region. A pulse transformer or other source of pulsed high voltage is also provided externally of the low pressure region and connected to the electrode structure to periodically apply accelerating voltage thereto. Electrons emitted from the cathode are accordingly periodically accelerated to an energy commensurate with the voltage of the pulses applied to the electrode structure to thereby provide a pulsed beam of energetic eleetrons. The energy and current of a pulsed electron beam obtainable from conventional sources or electron guns of the foregoing type are respectively relatively low where satisfactory pulse shapes and rise times are additionally requisites due to various material and design limitations. More particularly, the beam energy and current are generally limited to values of the order of hundreds of kiloelectron-volts and tens of amperes respectively, due to the attendant problems encountered in adequately insulating the windings of the pulse transformer from each other and from the electrode structure. In order to produce pulsed electron beams having energies and currents of the order of an order of magnitude greater than the above values by sealing of a conventional electron gun, a high strength dielectric, such as oil, is generally required to i lsulate the windings of the transformer. An extremely high voltage bushing, serving also as a vacuum seal, must in addition be employed between the dielectric of the transformer and the low pressure region of the electrode structure. Even with the formidable problem of fabricating the required high voltage bushing overcome, the necessarily high dielectric constant of the transformer insulating material produces a correspondingly large shunt capacity in the secondary winding thereof which is seriously limiting upon the shape and rise time of the output electron pulsrs from the gun. Accordingly, in applications which require relatively precisely formed fast rise time pulses of electrons at currents of the order of a hundred amperes and energies of the order of several mev., it has been heretofore necessary to V employ a conventional relatively low energy pulsed electron gun in combination with an auxiliary charged particle accelerator to produce pulsed electron beams having the indicated characteristics.

The present invention overcomes the foregoing limitations and disadvantages by providing a pulsed electron gun wherein the windings and'cores' of a novel pulse transformer are disposed within a low pressure region together with an electron emissive cathode and accelerating electrode structure. The transformer and electrode structure are capable of developing a pulsed electron accelerating field of at least 1 mev. intensity with substan- United States Patent 0 2,931,939 Patented Apr. 5, 1960 tially no insulating material other than vacuum being required between the transformer windings. In addition, a relatively large cathode surface may be employed to generate large electron currents of the order of amperes. Since vacuum (which has a dielectric constant of unity) is employed as the principal insulating material between the transformer windings, the high energy, high current electron pulses are produced in the electron gun of the present invention with relatively fast rise times.

The electron gun of the present invention is accordingly useful for many applications of electronics and nuclear physics wherein pulsed electron beams having the characteristics described above are required. For example, the electron gun may be employed directly as an extremely powerful source of ionizing radiation having extensive application in the large batch irradiation processing of various materials, such as plastics, for the purpose of improving various properties thereof; and foods and drugs to sterilize same. in addition the electron gun is specifically useful as a source of electrons in the apparatus disclosed in the copending application, Serial No. 337,994, of N. Christofilos.

it is an object of the present invention to provide an extremely high current high energy pulsed beam of electrons wherein the pulses have extremely fast rise times.

It is another object of the present invention to provide an electron gun wherein the windings and core structure of the pulse transformer for developing accelerating voltage are all disposed Within the vacuum region of the electron beam.

Still another object of the present invention is the provision of a pulsed electron gun wherein magnetic field effects on the electron beam due to cathode heater current flow are eliminated.

Yet another object of the invention is the provision of a pulsed electron gun including an accelerating voltage developing pulse transformer having minimized shunt capacity of the secondary winding thereof.

A further object is to provide a pulsed electron gun including an electrically solid anode in the form of a dense grid for the substantially parallel termination of an accelerating field shaped to minimize transverse acceleration of electrons and to produce a high quality focused beam.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in conjunction with the accompanying drawings, of which:

Figure l is a cross sectional plan View of a preferred embodiment of the invention;

Figure 2 is a cross sectional view of this embodiment taken along the line 22 of Figure 1; and

Figure 3 is a. schematic wiring diagram of preferred circuitry for energizing the components of this embodirnent.

Considering now the invention in some detail and referring to the illustrated form thereof in the drawings, there is provided an electron gun 11 including means for establishing a high vacuum region as shown generally at 12. Within vacuum region 12 is disposed novel generally annular high voltage pulse transformer structure 13 for developing a pulsed high voltage in the secondary winding in response to relatively low voltage pulses applied to the input thereof. Electron source means 14 are concentrically disposed within the transformer structure at one end thereof for emitting electrons in copious quantities. Accelerating electrode structure 15 including a plurality of coaxially spaced electrodes is also provided extending coaxially from the cathode 14 through the transformer structure 13 to the opposite end thereof. The electrodes are respectively connected to equal voltage increment tapping points of the secondary winding of the transformer structure to establish a high potential pulsed electron accelerating field extending axially through the transformer structure 13 from the cathode 14. The copious quantities of electrons emitted from the cathode are accordingly periodically accelerated to high energies by the "pulsed field to thereby provide a high energy, high current pulsed electron beam.

The means for establishing the high vacuum region 12 of previous mention is facilitated as by means of a holow cylindrical vacuum tank 16. The vacuum tank is preferably disposed in an elevated vertical position and formed as a circular base plate 17 (see Figure 2) supported above floor level as by means of vertical members 20 and having elongated upright cylindrical wall structure 18 attached integrally therewith. The open end of wall structure 18 is preferably flanged to facilitate pressure sealed attachment of removable closure plate means 19 thereto which affords ready access to the interior of the tank 16 for purposes of mounting the electron gun components therein. Base plate 17 includes a central axial aperture 21 to facilitate open beam outlet structure for communicable connection to the vacuum system of auxiliary beam utilization apparatus or in some applications to provide means for mounting a beam output window (not shown) in pressure sealed relationship therein. In addition, an anode grid may in some instances be disposed within aperture 21 for purposes which are subsequently described. The interior of the closed tank 16 may accordingly be evacuated to establish the high vacuum region 12 therein by means of a vacuum pump (not shown) communicably connected thereto, and to facilitate the foregoing a pumping port 22 is preferably provided in pressure sealed attachment to wall structure 18.

Considering now in detail the novel transformer structure 13 which is provided within high vacuum region 12, and referring to Figure 1, it is to be noted that such transformer structure is disposed concentrically within the cylindrical wall structure 18 of vacuum tank 16. More particularly, the transformer includes a plurality of rectangular laminated magnetic cores 23 which are radially disposed in circumferentially spaced relationship within vacuum tank 16 parallel to the interior surface of wall structure 18. Cores 23 are preferably arranged in two groups 24, 26 of equal number which occupy opposite 90 annular sectors of the circular cross sectional area en closed by vacuum tank 16. To accomplish this end, a pair of substantially 90 ring mounting segments 27, 28 are secured, as by means of brackets 29, in diametric opposition circumjacent the inner surface of wall struc ture 18 proximate the flanged upper end thereof. A plurality of radially disposed cinch straps 31 (see Figure 2) are provided which are adapted to engage the upper radial edge surfaces of cores 23 as arranged in the groups 24, 26 described above. The outer ends of the straps 31 are respectively secured in circumferentially spaced relationship to mounting segments 27, 28 by means of a plurality of elongated tie rods 32 secured at one end to the straps and extending axially through the segments. The opposite ends of tie rods 32 are in turn secured to a plurality of radially disposed saddles 33 which conform to the lower radial edge surfaces of cores 23 and are at tached to base plate 17 in respective axial alignment with cinch straps 31. The cinch straps 31 are additionally secured at their inner ends to saddles 33 by means of a second plurality of tie rods 34. Accordingly rigid mounting of the cores 23 within vacuum tank 16 is accomplished by adjusting tie rods 32, 34 to tighten the cinch straps 31 against the upper edge surfaces of the cores.

Magnetic flux is established in each one of cores 23 by energization of a primary winding 36 wound upon each core. In order to facilitate low impedance structure to the relatively large energizing currents and voltages which are employed in the present invention, primary windings 36 are best provided as relatively wide thin plate strips bent into concentric spirals respectively encircling the inner axial straight sections of the cores and secured in insulated relationship to tie rods 32. Each convolution (two convolutions illustrated in the drawing) of the spiral comprises a turn of the primary winding, and the ends of each primary winding so formed are respectively connected to the inner and outer conductors of one of a plurality of coaxial feed-through connectors 37 extending radially through the vacuum tank wall structure 18 in insulated pressure sealed relation therewith. Connectors 37 are circumferentially spaced in the wall structure 18 and terminate exteriorly thereof in conventional high voltage coaxial terminations to facilitate energization of the primary windings 36 from a suitable source of power in a manner which is subsequently described.

The magnetic flux established in each of the cores 23 upon energization of its associated primary winding 36 is employed in the present invention to induce an extremely high voltage in a secondary winding 38 of transformer 13 in serial fashion. To this end, secondary winding 38 is preferably provided as an axially elongated self-supporting helix disposed coaxially within vacuum tank 16 with each convolution of the spiral passing transversely through the windows, i.e., the central rectangularly apertured sections, of cores 23 in respective succession. The helical secondary winding 38 is preferably axially tapered in the direction of base plate 17 and formed, for purposes subsequently described, from tubing of a structurally strong material which is also suitable for high vacuum service, such as stainless steel. The entire winding 38 is supported within vacuum tank 16 in the position described above as by means of a suitable structural support member 39 secured to base plate 17 preferably at a position circumferentially intermediate the two groups 24, 26 of cores 23 and rigidly engaging the lowermost convolution of the winding. The rigidity of the secondary winding 23 and proper spacing of the adjacent turns thereof are, moreover, maintained as by means of appropriate insulated spacers 41 secured therebetween. The lower end of secondary winding 38 is led exteriorly of vacuum tank 16 through insulated feedthrough sealing means 42 which is best mounted in wall structure 18 at a point substantially radially opposite support member 39. The upper end of Winding 38 is terminated in a dome-shaped high voltage shield cap 43 which is disposed substantially coaxially of the vacuum tank 16 in the upper end "region thereof. In order to isolate the secondary winding 38 from the primary windings 36, a plurality of closed shields 44 are provided which conform generally'to the inner periphery of the cores 23 and are attached respectively thereto. Shields 44 thus comprise a closed electrical conduction path parallel to the magnetic flux paths through the cores and interposed between the primaries and secondary, thereby eliminating extraneous coupling effects therebetween.

Preferred structure for establishing an electron accelerating electric field axially 'of vacuum tank 16 from the voltage induced in secondary winding 38 comprises a plurality of graded cylindrical cup-shaped centrally apertured electrodes 46. Such electrodes are coaxially disposed in spaced, nested, mutually insulated relationship within secondary winding 38 to extend from high voltage cap 43 to base plate beam outlet aperture 21.

' The open faces of electrodes 46, moreover, are appropriately shaped such that, upon energization, the electric field thereby established is of a configuration to facilitate formation of a well-defined electron beam as is subsequently described. In the preferred embodiment, electrodes 46 correspond in number to the number of turns of secondary winding 38. Accordingly, to facilitate energization of the electrodes as well as rigid support thereof in the above-indicated relationship, a plurality of electrically conducting hanger rods 47 are provided which are respectively attached between the confvol utions of winding 38 and the electrodes 46. Hanger clamps 48 at axially aligned positions intermediate the groups 24, 26 of cores 23. The hanger rods then extend radially inward from the winding to be attached at their inner ends to electrodes 46 by means of suitable fasteners 49. Electrodes 46 are additionally supported by a plurality of insulated hanger rods 51 which are attached at their outer ends as by means of ring clamps 52 to the convolutions of winding 38 in diametric opposition to the conducting hanger rods 47. Hanger rods 51 extend radially inward from winding 38 to be attached at their innet ends to electrodes 46 by means of appropriate fasteners 53. With the electrodes 46 mounted and connected to the secondary winding 38 as described above it is to be noted that the overall voltage induced in the winding is tapped at successive points of substantially equal voltage for application to successive ones of the electrons whereby an electric field is established between adjacent ones thereof which increases incrementally in the direction of base plate 17. In order to provide optimum termination to the accelerating field, a suitable anode grid 46 may be advantageously mounted within beam outlet aperture 21 and grounded by electrical contact with base plate 17. Grid 40 preferably is constructed to have a relatively high transparency and because of the relatively high beam energy of the present invention such grid is best water cooled.

Within the first one of accelerating electrodes 46 is disposed the electron source 14 of previous mention. More particularly, source 14 preferably comprises a hollow cylindrical cathode housing 54 which is concentrically disposed in radially spaced relation within the first electrode and rigidly attached at its upper end to high voltage cap 43 as by means of an annular clamping member 56. An electron emissive cathode 57 is secured in closing relationship within the lower end of housing 54 adjacent the open end of the first one of accelerating electrodes 46 and a filament 58 is disposed within the housing adjacent the cathode to heat same upon energization and effect thermionic emission of electrons therefrom. Cathode 57 is accordingly preferably fabricated from a suitable thermionic emitting material and in this connection it has been found particularly advantageous in practice to employ a cathode constructed from a mixture of barium and strontium carbonates and nickel powder pressed onto a nickel backing plate. In order to minimize space-charge effects at the edges of the electron beam emitted from cathode 57, electron source 14 additionally includes an extractor electrode 59 having a generally similar configuration as the accelerating electrodes 46, i.e., a cylindrical cup-shaped centrally apertured electrode, and which is mounted concentrically about housing 54 in radially spaced insulated relation therefrom as by means of an annular mounting member 61. Extractor electrode 59 is accordingly concentrically interposed between housing 54 and the first one of accelerating electrodes 46 with the open end of such extractor electrode proximate cathode 57. Extractor electrode 59 moreover is connected to a suitable auxiliary source of extraction voltage, as is subsequently described in detail, in order to establish an electron extraction field adjacent cathode 57 which facilitates extraction of electrons therefrom as a well-defined beam prior to introduction to the accelerating field established by electrodes 46. In order to isolate the extraction and accelerating fields from transformer 13 and vacuum tank 16, the accelerating electrode structure 15 and electron source 14 are best disposed within a hollow cylindrical electrically conducting shield ing member 62 disposed concentrically between such elements and the transformer cores 23.

In order to facilitate connection of filament 58 and extractor electrode 59 to appropriate sources of electrical energy disposed externally of electron gun 11, electrical conductors may be connected thereto and led exteriorly of vacuum tank 16. More particularly a pair of lead-in conductors 63, 64 are preferably connected to one end of filament 58, the opposite end of which is connected to ground, and to extractor electrode 59. Conductors 63, 64 are then led through the hollow interior of the tubular secondary winding 38 in a bifilar manner to emerge from the winding exteriorly of vacuum tank 16 adjacent the secondary winding feedthrough bushing 42. Conductors 63, 64 may then be extended for connection to externally disposed electrical energy sources of a type described hereinafter.

Energization of the components of the above-described electron gun 11 is accomplished by circuits such as illustrated in the schematic wiring diagram of Figure 3 to enable these elements to perform their designated functions. As shown in the figure there is provided a pulse line 66 which is charged by a DC. power supply 67 connected in energizing relation thereto. The output of pulse line 66 is parallel connected through suitable high current switching means 68, e.g., a plurality of ball gaps, to input connectors 37 which are in turn connected to the primary windings 36 of transformer 13 as previously described. Switching means 68 is repetitiously actuated as by means of a trigger generator 69 connected thereto to thus repetitiously discharge pulse line 66 through primary windings 36 in parallel. With the characteristic impedance of pulse line 66 matching the impedance of primary windings 36, upon actuation of switching means 68 one half of the charging voltage of power supply 67 is simultaneously impressed across the primary windings while the remaining half of the voltage appears across the pulse line. The resulting currents flowing through the primary windings establish magnetic flux in the plurality of cores 23. Accordingly voltage is induced serially in the turns of the secondary winding 38 as such turns pass through the plurality of cores in succession. The overall voltage induced in the secondary winding is then applied incrementally to the plurality of accelerating electrodes 46 as previously described to establish the electron accelerating field axially of vacuum tank 16 between electron source 14 and beam outlet aperture 21.

To facilitate energization of electron source 14 and production of an electron stream for introduction to the accelerating field, a half-wave filament power supply 71 is connected to filament lead-in conductor 63 and a pulsed power supply 72 is connected to extractor leadin conductor 64. Pulsed power supply 72 is operated synchronously with trigger generator 69 whereby the extraction field and accelerating field are simultaneously established. Power supply 72 and trigger generator 69, moreover, are pulsed during the zero current portions of the half-wave cycle of filament power supply 71 in order to eliminate detrimental magnetic field effects on the electron beam due to the magnetic field established by current flowing through the filament 58. Thus the halfwave current from power supply 71 flowing through filament 58 heats cathode 54 resulting in the thermionic emission of electrons therefrom. During the alternate zero current portions of the filament current cycle, switching means 68 is actuated by trigger generator 69 to thereby discharge pulse line 66 through primary windings 36 and pulse transformer 13. The voltage induced in the secondary winding 38 of such transformer is in turn applied to accelerating electrodes 46 to establish the pulsed electron accelerating field axially between cathode 54 and anode grid 45 disposed in beam exit aperture 21. Simultaneously with the pulsing of transformer 13 and establishment of the accelerating field, power supply 72 pulses extractor electrode 59 to establish the pulsed extraction field proximal cathode 54 whereby the electrons emitted therefrom are extracted as high current well-defined pulses of electrons and introduced to the accelerating field. Such high current pulses of electrons are consequently accelerated to extremely high energy in the accelerating field to thereby emerge from aperture 21 as a high current, high energy pulsed electron beam.

The operation of the novel transformer 13 of the present invention in establishing the extremely high voltage accelerating field will be better understood upon consideration of the ensuing specific example wherein a l mev. field is produced. In the example, twenty electrically paralleled primary windings 36 having two turns each are respectively wound upon twenty cores 23 and secondary winding 38 is provided as having eight turns. Accordingly there is a 4 to l step-up ratio per core between each primary winding 36 and the secondary winding 38. Since the secondary winding 38 passes through the twenty cores 23 in succession, the resulting overall step-up ratio for the twenty cores is 80 to 1. Thus for each volt applied to the parallel primary windings 36, 80 volts are induced between the ends of secondary winding 38. With a 25 kv. supply employed as power supply 67 for charging pulse line 66 to a like voltage and the impedance of the pulse line matching that of the primary windings 36, actuation of switching means rcsuits in the application of 12.5 kv. across the parallel primary windings. By virtue of the overall 80 to 1 step up ratio, 1 mv. is induced in secondary Winding The turns of the secondary winding are respectively conne" ed to the like number of electrodes 46 and therefore the overall 1 mv. is applied to the electrodes in a di ted manner with a gradient of substantially 125 kv. o co adjacent electrodes. The field gradient between the and last ones of electrodes 4-6 is thus 1 rnv. and QCC; ates electrons to 1 mev. energy.

While the invention has been disclosed with respect to a single preferred embodiment, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention. In this connection, it is to be appreciated that alternative structure from that hereinbefore described may be employed to support electrodes 45 and insulate adjacent convolutions of secondary winding 38. For example, insulated spacers 41 and insulated hanger rods 51 may be dispensed with and rigid high voltage insulating structure. of various configurations familiar to those skilled in the art alternatively attached between adjacent ones of electrodes 46. Thus it is not intended to limit the invention except as defined in the following claims.

What we claim is:

1. A device for producing a high energy pulsed electron beam comprising a vacuum tank enclosing a high vacuum region and having an axial aperture therein, a plurality of radially disposed circumferentially spaced magnetic cores having centrally apertured windows therein mounted within said tank concentrically about the axis thereof, a plurality of primary windings each wound upon a different one of said cores, said primary windings connected in electrical parallel, an axially elongated helical secondary windlng disposed coaxially within said vacuum tank with the convolutions of said winding passing transversely through the windows of said cores in circumferential succession, a plurality of closed electrically conducting shields conforming to the inner apertured periphery of said cores and respectively secured thereto to isolate said secondary winding from said primary windings, a pulsed power supply connected to said primary windings to induce pulsed voltage in said ondary winding, an electron source disposed Within said vacuum tank coaxially at one end of said secondar winding, and a plurality of accelerating electrodes disposed coaxially within said secondary winding and extending from said source to said aperture, said electrodes connected electrically to said secondary winding to establish apulsed electron accelerating field axially between said source and said aperture whereby a high energy pulsed electron beam emerges from said aperture.

2. A pulsed electron gun-comprising a hollowcylmdrical vacuum tank enclosing a high vacuum region and having a central axial aperture at one end thereof, a plurality of rectangular magnetic cores having centrally apertured windows therein, said cores radially disposed in circumferentially spaced relationship within said vacuum tank concentrically about the axis thereof, a plurality of primary windings each wound upon a different one of said cores, said primary windings connected in electrical parallel, an axially elongated helical secondary winding coaxially disposed within said vacuum tank with the convolutions of said winding passing through the windows of said cores in circumferential succession, said secondary winding being formed of a tubular conductor and the proximal end thereof relative to the vacuum tank aperture extending exteriorly through the wall of said vacuum tank in pressure sealed relation thereto, a pulsed voltage source connected to said primary windings to cyclically induce pulsed voltage in said secondary winding, a plurality of accelerating electrodes extending in coaxially spaced relationship through the convolutions of said secondary winding to said aperture, said electrodes electrically connected to said secondary winding to establish a pulsed electron accelerating field axially therethrough in the direction of said aperture, an electron source disposed coaxially adjacent said electrodes at the distal end of said secondary winding relative to said aperture, said source including an electron emissive cathode and filament adjacent the cathode for heating same, and a pair of conductors connected to the respective ends of said filament and led bifilarly through the hollow interior of said secondary winding conductor to emerge from the lower end thereof exteriorly of said vacuum tank for connection to a filament power supply, whereby electrons emitted from said source are periodically accelerated through said accelerating field to emerge from said aperture as a high energy pulsed electron beam.

3. A device for producing pulsed beam of energetic electrons comprising a hollow cylindrical vertically elongated vacuum tank enclosing a high vacuum region and having a central axial aperture at the lower end thereof, means supporting said tank in an elevated position, a plurality of rectangular laminated magnetic cores having centrally apertured windows therein, said cores radially disposed and secured in circumferentially spaced relationship to the inner wall surface of said tank, a plurality of primary windings each wound upon a different one of said cores, a plurality of connector means mounted in insulated sealed relationship in the wall of said tank and extending externally thereof, said connector means respectively connected to said primary windings to provide external input terminals thereto, an axially elongated self-supporting helical secondary winding mounted coaxially within said vacuum tank with each convolution of the winding passing transversely through the windows of cores in circumferential succession, said secondary winding being formed from a tubular conductor and the ion er end thereof extending exteriorly through the wall of said vacuum tank in pressure sealed relationship thereto, a pulsed voltage source connected in parallel to said pluraiity of conne or means to cyclically induce pulsed voltage in said secondary winding, a plurality of graded centrally apertured cylindrical cup-shaped accelerating electrodes disposed in insulated axially spaced nested relationship and extending coaxially through the convolutions of said secondary winding to a position proximate said aperture, said electrodes mechanically and electrically connected to said secondary winding at successive equal voltage tapping points, a cylindrical housing concentrically disposed in radially spaced relation within the upper one of said electrodes, a high voltage cap mechanically and electrically connecting the upper end of said housing to the upper end of said secondary winding with the interior of the secondary winding conductor communicating with the interior of said housing,

an electron emissive cathode mounted within the lower end of said housing, a filament mounted within said housing adjacent said cathode, one end of said filament connected to ground, a centrally apertured cylindrical cupshaped extractor electrode concentrically interposed between the upper one of said accelerating electrodes and said housing and attached in insulated relationship thereto, a pair of electrical conductors respectively connected to said filament and said extractor electrode, said conductors led bifilarly through the hollow interior of said secondary winding conductor to emerge from the lower end thereof exteriorly of said vacuum tank, a filament power supply connected to said conductor connected to said filament, and an extractor pulsed power supply connected to said conductor connected to said extractor electrode whereby a high energy pulsed electron beam is transmitted through said aperture.

4. A device as defined by claim 3 but wherein said filament power supply is a half-wave supply and said pulsed voltage source and said extractor pulsed power supply are synchronously operated during the alternate zero current portions of the half-wave cycle of said filament power supply.

5. A high voltage pulse transformer comprising means for establishing a high vacuum region, a plurality of rectangular magnetic cores having centrally apertured windows therein, said cores radially disposed in circumferentially spaced relationship within said region, a plurality of primary windings each wound upon a difierent one of said cores, said primary windings connected in electric parallel and adapted for connection to a source of time varying voltage, an axially elongated helical secondary Winding disposed within said region with the convolutions of said winding passing transversely through the windows of said cores in circumferential succession, and a plurality of closed electrically conducting shields conforming generally to the inner apertured periphery of said cores and respectively secured thereto to isolate said secondary winding from said primary windings whereby high voltage pulses are induced in said secondary winding upon energization of said primary windings.

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